[Rd] formatting issue with gcc 9.3.0 on Ubuntu on WSL2

[Iñaki Ucar]

On Wed, 18 Nov 2020 at 10:26, Tomas Kalibera <tomas.kalibera using gmail.com> wrote:
>
> On 11/17/20 9:34 PM, Bill Dunlap wrote:
> > I just got a new Windows laptop (i7, 10th generation CPU), installed
> > 'Windows Subsystem for Linux 2' and then installed Ubuntu 20.04 and
> > used 'apt-get install' to install packages that the R build seems
> > to require.  In particular, I am using gcc version 9.3.0.   The
> > build went without a hitch but the tests showed that deparse(1e-16)
> > produced "1.00000000000000e-16" instead of the expected "1e-16".
> >
> > It looks like the problem is in src/main/format.c:scientific().  The
> > lowest two+ bytes in the fractional part of the long double (80-bit)
> > return value of powl(10.0L, -30L), seem to be corrupted.  I made a
> > standalong program to test powl and saw no problem - it gives the
> > same results for the fractional part as bc does.
> >
> >          bc: A2425FF7 5E14FC31 A125...
> > standalone: 22425FF7 5E14FC32
> >           R: 22425FF7 5E151800
> >
> > There are lots of other small numbers with the same problem:
> >
> >
> >                               > grep(value=TRUE, "0e",
> > vapply((1+(0:10000)/1000)*1e-15, deparse, ""))
> > [1] "8.56000000000000e-15" "8.71700000000000e-15" "8.77800000000000e-15"
> > [4] "8.93500000000000e-15" "9.50800000000000e-15" "9.83800000000000e-15"
> > [7] "9.89900000000000e-15" "9.93400000000000e-15" "9.99500000000000e-15"
> >> str(grep(value=TRUE, "0e", vapply((1+(0:10000)/1000)*1e-14, deparse, "")))
> >   chr [1:295] "8.00200000000000e-14" "8.00500000000000e-14" ...
> >
> > Has anyone else seen this?  I am wondering if this is an oddity in WSL2
> >
> >                               or Ubuntu's gcc-9.3.0.

I cannot reproduce this issue (version 20H2, build 19042.630; Ubuntu
20.04 installed from the store). Are you sure you are running on WSL2?
(You can check this with `wsl --list --verbose`).

> Almost surely it is Windows/WSL related, I'm not seeing this on Ubuntu
> 20.04.
>
> One thing to check might be the FPU control word. In a Windows build, R
> will set as it is on Unix, to use all 80 bits when values stay in FPU
> registers, which is not the Windows default. This should not matter with
> SSE anymore, but maybe something is still using the FPU. This is just
> using inline assembly, so one could enable it as experiment. In
> principle, this could be also due to some other things specific to
> Windows that R works around in Windows builds, but doesn't in Linux
> builds assuming they will not run on Windows.

It does run on Linux. WSL2 runs a modified version of the Linux kernel
on top of Hyper-V. Unless Bill is running WSL1, which runs on top of
the Windows kernel with a syscall translation layer.

> Other issues I had with WSL in the past (trying to build R and run
> checks) included time-zones and surprising encodings, but I didn't check
> recently. I would not use R on WSL unless my goal was to diagnose these
> issues and see if they could be overcome on the R side.
>
> Best
> Tomas

-- 
Iñaki Úcar